The present invention relates to a process for the preparation of pure alkyl tert.-alkyl ethers and hydrocarbon raffinates which are largely free from isoolefins and from alkanols, by reaction, on acid ion exchangers, of alkanols and hydrocarbon mixtures containing at least one tert.-olefin, and absorptive removal of excess alkanol from the reaction mixture.
Alkyl tert.-alkyl ethers, such as methyl tert.-butyl ether (MTBE) and tert.-amyl methyl ether (TAME), are used, for example, as fuel additives for carburettor-type engines for improving the octane number, and as solvents and extracting agents which, for example, form hardly any peroxides, in contrast to many other ethers. In addition, the alkyl tert.-alkyl ethers are suitable starting materials for the preparation, via ether cleavage, of pure tert.-olefins based on these ethers.
The reaction of tert.-olefins and alkanols on acid ion exchangers to give alkyl tert.-alkyl ethers is known (German Pat. No. 1,224,294). Since this reaction is very selective, mixtures of tertiary olefins with other hydrocarbons, as produced, for example, in thermal or catalytic crackers, are particularly suitable starting materials.
The closeness of approach of the tertiary olefin conversion to the quantitative conversion depends on the type of tertiary olefin and on the choice of the alkanol. In this respect, the conversions of i-butene, for example from the C.sub.4 -raffinate of thermal cracking (mostly called C.sub.4 -raffinate I), are particularly high, whilst the conversions of i-amylene and other tertiary olefins of increasing carbon number are lower under comparable conditions. Conversion and yield can be improved by special measures in carrying out the reaction during the preparation of MTBE, such as the choice of 2 or more successive reaction stages (German Offenlegeschrift No. 2,521,964, German Offenlegeschrift No. 2,706,465, and German Offenlegeschrift No. 2,944,914), or the use of a large excess of methanol (German Offenlegeschrift No. 2,853,769).
Particularly in the case of C.sub.4 -raffinates, a high i-butene conversion is desirable, since, for further utilization of the i-butene-free C.sub.4 -raffinate, the residual content of i-butene should be below 2% by weight, if possible even below 1 or below 0.5% by weight. This specification is necessary when, for example, maleic acid anhydride, methyl ethyl ketone, but-1-ene or octenes are to be prepared, by dimerization, from these residual mixtures, mostly called C.sub.4 -raffinate II. For further use in this respect, care must also be taken to ensure that alkanol is no longer present in the C.sub.4 -raffinate II. Thus, achieving an "on-specification" raffinate and a pure ether, such as, for example, in the processes for the production of MTBE and TAME, involves special requirements.
However, owing to azeotropic effects, it has been found that the two required aims, namely a methanol-free ether and also a methanol-free C.sub.4 -raffinate II or C.sub.5 -raffinate, cannot be achieved by simple distillation (see German Offenlegeschrift No. 2,802,198, page 3, paragraph 3). C.sub.4 - and C.sub.5 -hydrocarbons in particular form methanol-containing azeaotropes which, when i-butene/i-amylene are etherified together, for example from hydrocarbons which contain various C.sub.4 - and C.sub.5 -olefins and -alkanes, made an exact C.sub.4 /C.sub.5 -separation impossible, since, together with the methanol, C.sub.5 -hydrocarbons also occur in the C.sub.4 -top product of the debutanizing column.
An azeotrope is also formed, for example, by MTBE and methanol. A water wash has been proposed (German Offenlegeschrift No. 2,246,004 and U.S. Pat. No. 3,726,942) as a useful method for removing the methanol from the C.sub.4 -raffinate II and from the MTBE/methanol azeotrope. Additional suitable methods for separating the MTBE/methanol azeotrope are extractive distillation using dimethylsulphoxide (Japanese Preliminary Published Application 73-00509) or using pentane (U.S. Pat. No. 3,940,450) or distillative separation of the azeotrope from pure MTBE using relatively high pressures of 5 to 20 bar (German Offenlegeschrift No. 2,629,769).
French No. 7,903,416 describes the separation of methanol from the C.sub.4 -raffinate II as an absorption process on a molecular sieve, with subsequent desorption by hot nitrogen and recovery of the methanol by condensation from the N.sub.2 stream.
The stated measures for the purification of the reaction products and raffinates require substantially higher capital costs than the actual etherification reaction, and also require a large additional energy demand, relative to the total process.
Furthermore, U.S. Pat. No. 3,409,691 discloses the separation of an amount of 500 ppm of secondary butanol from n-hexane on macroporous cation exchangers in the Na.sup.+ form. To regenerate the cation exchanger, the butanol is displaced by methanol and the methanol is in turn removed again by heating the cation exchangers at 110.degree. C. in vacuo overnight.
Furthermore, German Offenlegeschrift No. 2,027,066 describes the possibility of removing methanol from water (1 g of methanol per 1 l of water) with the aid of a styrene/divinylbenzene polymer containing nitro groups. The adsorptive power in this case is 18 g of methanol per kg of adsorbent. The adsorbent is regenerated by means of steam distillation.